Evolution and Natural Selection Flashcards

Species Definition

  • Defining a "species" isn't straightforward and involves three concepts that may not always align.

Biological Species Concept

  • Defines species as organisms that can interbreed and produce fertile offspring.
  • Different-looking organisms can belong to the same species if they produce viable offspring.
  • Offspring from such interbreeding are often called "hybrids.”

Morphological Species Concept

  • Defines species as organisms that look alike and share morphological traits.
  • Identical-looking organisms may belong to different species if they cannot produce offspring.

Phylogenetic Species Concept

  • Defines species as organisms that share identical gene sequences.
  • Considered the most accurate in evolutionary terms, as it elucidates the degree of relatedness among organisms.

Macroevolution vs. Microevolution

  • Evolutionary study is divided into two general areas.

Macroevolution

  • The study of evolutionary changes at the level of anatomical features.

Microevolution

  • The study of evolutionary changes at the genetic (molecular) level.

Common Misconceptions About Evolution

  • Individuals do not evolve; populations evolve.
  • Evolution occurs at the population level.
  • The study of microevolution at the population level is termed population genetics.

The Modern Synthesis

  • Population genetics studies how populations change genetically over time.
  • It integrates Mendelian genetics with Darwin's theory of evolution by natural selection.
  • This synthesis focuses on populations as units of evolution.

Gene Pools and Allele Frequencies

  • A population is a localized group of individuals capable of interbreeding and producing fertile offspring.
  • The gene pool is the total aggregate of genes in a population at any given time.
  • It consists of all gene loci in all individuals of the population, including all alleles.

The Hardy-Weinberg Theorem

  • Describes a population that is not evolving.
  • States that allele and genotype frequencies in a population's gene pool remain constant from generation to generation, provided that only Mendelian inheritance and recombination are at work.
  • Mendelian inheritance preserves genetic variation, while natural selection can remove variation.

Allele Frequencies and Hardy-Weinberg Equilibrium

  • Allele frequencies are calculated for both males and females in a population.
  • If p represents the dominant allele frequency and q represents the recessive allele frequency, then (p + q) = 1.0.
  • Algebraic simplification leads to the equation p^2 + 2pq + q^2 = 1, where:
    • p^2 is the frequency of the homozygous dominant genotype.
    • 2pq is the frequency of the heterozygous genotype.
    • q^2 is the frequency of the homozygous recessive genotype.
  • If allele frequencies are not undergoing selection, the total allele frequencies of p and q will equal 1 in subsequent generations, indicating Hardy-Weinberg equilibrium.

Hardy-Weinberg Equilibrium and Selection

  • Example: Flower color controlled by alleles R (red) and r (white).
    • Generation 1: All plants are Rr (pink flowers).
    • Generation 2: RR, Rr, and rr genotypes appear in a 1:2:1 ratio due to random segregation of alleles.
  • If no selection occurs, these allele frequencies remain the same across generations; this is modeled by the Hardy-Weinberg Theorem.
  • If 500 plants are counted (1000 alleles), with 320 RR (red), 160 Rr (pink), and 20 rr (white):
    • Frequency of R allele = (320 "," 2 + 160 "," 1)/1000 = 0.8 (80%).
    • Frequency of r allele = (160 "," 1 + 20 "," 2)/1000 = 0.2 (20%).
  • If red flowers are selectively eaten:
    • New allele frequencies are calculated based on the remaining pink and white plants.
    • If the calculated frequencies deviate from the Hardy-Weinberg equilibrium, it indicates that selection has occurred.
  • For example, if only 180 pink and white plants remain (360 alleles) after deer eat the red flowers:
    • R(p) = 180/360 = 0.5
    • r(q) = 20/360 = 0.06
    • Then, p^2 + 2pq + q^2 = 0 + 2(0.5)(0.06) + 0.06^2 = 0.096.
    • The population is no longer in Hardy-Weinberg equilibrium, suggesting selection.

Rules for Hardy-Weinberg Equilibrium

  • If any of these rules are broken, allele frequencies will change, and evolution occurs:
    • Extremely large population size: Smaller populations increase the chance of genetic drift, reducing genetic diversity.
      • If broken: Genetic drift.
    • No Gene Flow: Transfer of alleles from inter-population matings can alter allele frequencies.
      • If broken: Introduction of new alleles.
    • No Mutations: Changes in chromosome structure can reproductively isolate members, preventing viable offspring.
      • If broken: Postzygotic isolation.
    • Random Mating: Preferential mating alters gamete mixing.
      • If broken: Sexual selection.
    • No Natural Selection: Selective pressure determines differential reproductive success, altering allele frequencies.

Genetic Drift and Bottleneck Effect

  • Genetic drift changes allele frequencies over time without selection.
  • A genetic bottleneck is similar but results from massive mortality in the mating population.

Selection Types

  • Selection can change allele frequencies over time; common types exist.
  • Sickle Cell Anemia, Stabilizing Selection, and Human adaptation to the Malaria parasite are examples.

Natural Selection

  • Natural selection results from:
    • Success in reproduction.
    • Accumulation of habitat-specific traits in a population.
  • It adapts a population to its environment.
  • When the environment changes, natural selection also changes.

Reproductive Barriers and Speciation

  • Reproductive barriers restrict gene flow and can cause biological speciation.

Prezygotic Barriers

  • Gametic Isolation

Postzygotic Barriers

  • Reduced Hybrid Viability
  • Reduced Hybrid Fertility
  • Hybrid Breakdown

Evolution and Life's Diversity

  • Modern living species are descendants of older, often extinct species (99.1% of species that have existed are extinct).
  • All life shares degrees of relatedness; differences and similarities among species can be explained by descent with modification.
  • Descent with modification results from adaptation to the natural environment due to natural selection.
  • Adaptation and natural selection are key components of evolution.

Evolutionary Theory

  • Unifies the broad field of Biology.
  • An idea becomes a scientific theory when:
    • It can explain natural phenomena under different testing conditions.
    • It can guide further levels of questioning.

Resistance to Evolution

  • The Origin of Species challenged deeply rooted Western culture and a centuries-old worldview.

How Evolution Theory Evolved

  • Key figures and events that contributed to the development of evolutionary theory:
    • Linnaeus (classification)
    • Hutton (gradual geologic change)
    • Lamarck (species can change)
    • Malthus (population limits)
    • Cuvier (fossils, extinction)
    • Lyell (modern geology)
    • Darwin (evolution, natural selection)
    • Mendel (inheritance)
    • Wallace (evolution, natural selection)

Geology and Evidence Against Creationism

  • Geology provided evidence against creationism by suggesting Earth was much older than 6,000 years, evidenced by formations like the White Cliffs of Dover and the Grand Canyon.
  • Fossils indicated that many ancient organisms had gone extinct.

Darwin's Contribution

  • Darwin's major points in Origin of Species:
    • Modern organisms are descendants of ancient organisms.
    • Natural selection results in evolution, defined as change over time, due to adaptation to environments.
    • Origin of Species took forty years to write, based on meticulous analysis of overwhelming evidence.

Darwin's Voyage on the HMS Beagle

  • Darwin was a naturalist on the HMS Beagle, tasked with collecting and cataloging biodiversity in South America.

Darwin's Observations in the Galapagos

  • Darwin noted that Galapagos species were similar but not identical to those in mainland Ecuador.
  • The Galapagos Islands, formed by volcanic activity, provided new habitats for colonization.

Galapagos Marine Iguana

  • Specialized adaptations of Galapagos marine iguanas:
    • Jaws and skull for scraping algae.
    • Limbs and tail for swimming.
    • Black coloration for camouflage and heat absorption.
    • Fat content for thermoregulation.
  • Comparison with arboreal iguana (ancestral state):
    • Jaws and skull for accessing fruit and hunting insects.
    • Limbs and tail for climbing trees.
    • Coloration for camouflage in forests.
    • Fat content for quick energy reserves.

Galapagos Finches

  • Descent with modification from ancestral species Geospiza fortis on the mainland.
  • Examples of finch species and their adaptations:
    • Cactus eater (Geospiza scandens): long, sharp beak.
    • Seed eater (Geospiza magnirostris): large beak.
    • Insect eater (Certhidea olivacea): narrow, pointed beak.

Adaptive Radiation in Galapagos Finches

  • When an ancestral species colonizes a new habitat, adaptive radiation can occur.
  • Examples: medium tree finch, small tree finch, vegetarian finch, mangrove finch, woodpecker finch, cactus finch, sharp-beaked ground finch, warbler finch
  • Each finch species adapted to different food sources and habitats.

Artificial Selection

  • Human selective breeding has produced descent with modification (artificial selection).
  • Example: Six varieties of the wild mustard plant through selective breeding.
    • Cabbage
    • Brussels sprouts
    • Kale
    • Kohlrabi
    • Cauliflower

Natural Selection and Adaptation

  • Mantid species from different continents (flower mantid in Malaysia, stick mantid in Africa) share a common ancestor.
  • Natural selection results in vastly different coloration patterns.
  • Populations adapt to their habitats, blending in with vegetation to obscure themselves from predators and prey.

Convergent Evolution

  • Similar selective pressures cause similar adaptive traits in different lineages (convergent evolution).
  • Example: Sugar glider (Australia) and flying squirrel (North America).

Homologous Characters

  • Characters descended with modification from a common ancestor.
  • Example: Vertebrate appendages of the pectoral girdle (human, cat, whale, bat).
    • humerus
    • ulna
    • radius
    • carpals
    • phalanges

Vestigial Appendages

  • The tails of human embryos.
  • The hind limbs of ancient whales.

Darwin's Main Ideas

  • Evolution explains life's unity and diversity.
  • Natural selection causes adaptive evolution.

Descent with Modification

  • Summarizes Darwin's perception of the unity of life.
  • All organisms are related through descent from a common ancestor.
  • The history of life is like a tree with branches representing life's diversity.

Phylogeny and Evolutionary History of the Elephant Family

  • Evolutionary relationships among different elephant species and their ancestors.
  • Examples: Hyracoidea, Sirenia, Moeritherium, Barytherium, Deinotherium, Mammut, Platybelodon, Stegodon, Mammuthus, Elephas maximus, Loxodonta africana, Loxodonta cyclotis

Natural Selection Experiment

  • Experimental transplant of guppies between pools with different predators (killifish and pike-cichlids).
  • Guppies in killifish pools are larger at sexual maturity.
  • Guppies in pike-cichlid pools are smaller at sexual maturity.

Results of Guppy Transplant Experiment

  • After 11 years, transplanted guppies shifted from being small to being large.
  • This demonstrates natural selection at work.

Ernst Mayer's Dissection of Darwin's Logic

  • Based on five observations:
    • Observation 1: Populations increase exponentially if all individuals reproduce successfully.
    • Observation 2: Populations tend to remain stable in size.
    • Observation 3: Resources are limited.
    • Inference 1: Struggle for existence leads to only a fraction of offspring surviving.
    • Observation 4: Members of a population vary extensively.
    • Observation 5: Most variation is heritable.
    • Inference 2: Survival depends on inherited traits; individuals with favorable traits have higher fitness.
    • Inference 3: Unequal ability to survive and reproduce leads to gradual change in a population with favorable characteristics accumulating.

Summary of Natural Selection

  • Natural selection is differential success in reproduction.
  • It results from environmental changes that favor adaptive traits and natural variation.
  • Over time, natural selection increases the adaptation of organisms to their environment.
  • If the environment changes, natural selection may result in adaptation to new conditions.